Imagine that your best friend has been replaced by an exact double, or that everyone you meet is really the same person wearing lots of disguises. These aren't just plots of The Prisoner episodes — they're all real mental delusions.

The various delusions I just described are examples of monothematic delusions, which are simply any condition in which a person's delusion is limited to a single topic. Delusional states, which are any instance where a person holds onto a strong belief despite overwhelming evidence to the contrary, are common in mental illnesses like schizophrenia or bipolar disorder — indeed, they're so common that such patients will generally suffer from a whole bunch of different delusions, all adding together into one collective break from reality.

These monothematic delusions are a different beast. Someone suffering from one of these conditions will otherwise see the world more or less completely normally — it's only on this one point that they differ from the agreed upon nature of reality. Their intellect and other cognitive functions won't be otherwise impaired, and it's possible that they can even recognize how strange their convictions are, and yet they still can't manage to shake them.

The Unfamiliar Faces

The first of the delusions, in which a person becomes convinced impostors have replaced their friends and family, is known as the Capgras delusion, named for the French psychiatrist Joseph Capgras who along with Jean Reboul-Lachaux first described the condition in 1923. (If you're wondering why it's not known as the Reboul-Lachaux delusion instead, I need only mention that one was the other's intern.)

Their case study described a 53-year-old Parisian seamstress who had become convinced those around her were being kidnapped by strange creatures known as "sosies", which imprisoned her loved ones underground as they plotted to steal all her property:

Her husband [...] also disappeared: a sosie took his place; she wanted to divorce this sosie; she drew up a complaint and requested a separation from the court. Her real husband was murdered and the "gentlemen" who come to visit her at the hospital are "sosies" of her husband; she counted at least eighty of them.

"If this person is my husband, she says, he is totally unrecognizable, he is transformed. I certify that this so-called husband that they are trying to foist on me in fact ceased to exist ten years ago. To replace my stolen daughter, they always put another one who was in turn removed and immediately replaced ... Whenever they took away a child, they gave me another one who looked like her. I had over two thousand in five years [...]. Every day girls came to my home and every day they were taken away; I warned the Police Superintendent, saying that their parents had disappeared and that these girls had pricks to the face to remove all their ideas."

Capgras and Reboul-Lachaux noted that everyone the woman knew — including the entire staff of the hospital where she was being treated — had supposedly been replaced by these sosies. Their paper speculated that something had gone very wrong in the patient's face recognition, with her losing any sense of familiarity when she saw those she thought she knew. This discord between her thoughts and her feelings led her eventually to assume those around her were impostors, an evolving state that was "an emotional state first, then a habit, finally an automatic state of mind."

The World's Greatest Master of Disguise

Leopoldo Fregoli was one of the greatest entertainers of the late nineteenth and early twentieth century. He perfected a style of performance known as quick-change, which is pretty much exactly what it sounds — he would switch costumes and characters during his stage shows so rapidly that it was suggested he actually required several other Fregolis for his act to be possible. And while that notion of hidden duplicates would fit right in with the delusions we've been discussing, that's not actually what won him his unlikely — and quite possibly unwanted — immortality in the annals of psychology.

In 1927, French psychiatrists Paul Courbon and Gustave Fail described the case of a 27-year-old servant who had become convinced she was being persecuted by a pair of actresses named Sarah Bernhardt and Robine who she had seen at a nearby theater. The servant believed these two quick-change artists were impersonating everyone she met. It seems the servant saw this as more supernatural than true quick-change trickery, as she thought that they possessed the ability to place other people inside the bodies of those around her, and that they could actually get inside her mind, stealing her thoughts and forcing her to do things. Still, the quick-change connection stuck, and the greatest of them all gave his name to this delusion.

The Capras and Fregoli delusions are just two of a whole range of these strange delusions. Paul Courbon and his new collaborator Jean Tusques described intermetamorphosis in 1932, described as "the belief that the body and soul of different people are incarnated in the body of a single person." Then there's 1903's reduplicative paramesia, in which a person becomes convinced that a location has been duplicated and they are in the replica, which may be located far away from the original. According to Czech neurologist Arnold Pick, his patient was sure her hospital had been duplicated, and that Pick must simply work at both hospitals to explain how she knew him from both locations.

The Origins of Delusions

So what causes these bizarre breaks from reality? Certainly, they aren't unknown in mental disorders like schizophrenia or dementia, but when they appear as isolated phenomena, it's typically as a result of brain trauma, stroke, or neurological illness. Brain lesions are often pointed to as a potential physical culprit for the delusions, but as with most things about the brain, it's hard to say for certain.

It's a bit easier to trace the cognitive side of the problem. These delusions are all about problems in identification, albeit of two opposite types - the hyper-identification of the Fregoli delusion, in which a person obsesses on resemblances between people and becomes convinced they're all the same individual, and the hypo-identification of the Capgras delusion, in which tiny apparent changes in a person's appearance becomes evidence of impostors.

Once these mistaken identifications start occurring, the brain then needs to take the next big step by actually deciding these errors are actually indicative of a larger reality. Not everyone will make that leap, and there's some research to suggest a profile of those most susceptible to these delusions once presented with the apparent discrepancies.

Typically, these are people who are prone to jumping to conclusions and have a tendency to ignore any evidence that conflicts with their snap judgment. They can become convinced that a world full of impostors and body-jumping actors makes more sense than a world in which they are wrong. Unfortunately, that delusion seems to have become all too commonplace.